Nematic illumination

Besides the direct electrical induction of electro-optical effects in liquid crystals, their activation by illumination of photoconductors could be of great technical interest. This method might well permit the electro-optical properties of nematic liquid crystals to be used on a larger scale, because photoconductor activation may eventually be applied to light amplification, optical data processing, and projection display systems, or used for recording phase-type holograms. 

In a uniaxial nematic liquid crystal, the spatial orientation of the optical axis is determined by the orientation of the director. Due to thermally excited orientational director fluctuations, the spatial direction of the optical axis is not constant in time. As a result, any light illuminating the sample is... 

Fig. 4.8. Relaxation time r for the fundamental splay fluctuation mode as a function of sample thickness (circles) and best fit of the theoretically predicted relation for weak anchoring (solid line). The aligning layer was UV illuminated photoactive poly-(vinyl-cinnamate), the liquid crystal was 5CB in the nematic phase (T = 32°C) [59].

The threshold intensity for the laser induced deformation of nematic liquid crystals is calculated as a function of the spot size of the illuminating beam. It is shown, that due to the small size of the light spot, the optical threshold is higher than that expected from the Fredericks threshold formula for static electric fields of infinite large dimension in the cell plane. Experimental data measured on a homeotrop OCB (octyl-cyano-byphenyl) sample fit well the theoretical curve. Results obtained on the temperature and wavelength dependence of the threshold can be interpreted on the basis of the given theory. 

We investigate a homeotropic sandwich-like cell of a nematic liquid crystal (NLC), in which the L director is fixed uniformely at the boundaries, and which is illuminated with a linearly polarized and focused laser beam. It is assumed that the focal length of the focusing lens is large enough so that the laser beam can be considered as a parallel beam within the sample. 

For the experimental verification of the curve corresponding to the formula (3), two measurements were carried out. In the first exploratory experiment a homeotrop nematic MBBA cell of 150 jum thickness was illuminated with an argon ion laser beam of 65 /itn diameter (FWHM). At 21.5°C we got for the threshold 45 10 mW, while the theoretically expected value was 35 mW, which we found to be satisfactory, taking into account the simplifying assumptions and experimental uncertainties. 

Liquid crystals are generally characterized by the strong correlation between molecules, which respond cooperatively to external perturbations. That strong molecular reorientation (or director reorientation) can be easily induced by a static electric or magnetic field is a well-known phenomenon. The same effect induced by optical fields was, however, only studied recently. " Unusually large nonlinear optical effects based on the optical-field-induced molecular reorientation have been observed in nematic liquid-crystal films under the illumination of one or more cw laser beams. In these cases, both the static and dynamical properties of this field-induced molecular motion are found to obey the Ericksen-Leslie continuum theory, which describe the collective molecular reorientation by the rotation of a director (average molecular orientation). 

Liquid crystal displays (LCDs) have many advantages over other display types. They are flat and compact, possess extremely low-power consumption (microwatts per square centimeter in the case of the twisted nematic effect), their color and contrast does not fade with an increase in the illumination intensity, they work both in transmissive and reflective modes in a wide operating temperature range and with a long lifetime. Besides that, LCDs are the most economically produced flat display systems. LCDs have... 

---